1. Field of the Invention
This invention is concerned with upgrading residual petroleum fractions to selectively reduce CCR without undue hydrogen consumption. More particularly, the invention involves the use of hydrotreating with a novel catalyst to render residual fractions quite suitable as feedstocks in a subsequent coking operation.
2. Description of the Prior Art
Coking is one of the refiner's major processes for converting residuals to lighter, more valuable stocks. Petroleum coke is the residue resulting from the thermal decomposition or pyrolysis of high-boiling hydrocarbons, particularly residues obtained from cracking or distillation of asphaltenic crude distillates. The hydrocarbons generally employed as feedstocks in the coking operation usually have an initial boiling point of about 380.degree. C. (700.degree. F.) or higher, an API gravity of about 0.degree. to 20.degree., and a Conradson carbon residue content of about 5 to 40 weight percent.
The coking process is particularly advantageous when applied to refractory, aromatic feedstocks such as slurry decanted oils from catalytic cracking and tars from thermal cracking. In coking, the heavy aromatics in the resid are condensed to form coke. During coking, about 15-25 weight percent of the charge goes to form coke. The remaining material is cracked to naphtha and gas oil that can be charged to reforming and catalytic cracking.
Residual petroleum oil fractions such as those fractions produced by atmospheric and vacuum crude distillation columns are typically characterized as being undesirable as feedstocks for direct use in most refining processes. This undesirability is due primarily to the high content contaminants, i.e. metals, sulfur, nitrogen and Conradson carbon residue, in said fractions.
Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper also sometimes present. Additionally, trace amounts of zinc and sodium are found in most feedstocks. As the great majority of these metals when present in crude oil are associated with very large hydrocarbon molecules, the heavier fractions produced by crude distillation contain substantially all the metal present in the crude, such metals being particularly concentrated in the asphaltene residual fraction. The metal contaminants are typically large organo-metallic complexes such as metalloporphyrins and similar tetrapyrrole structures.
The residual fraction of single stage atmospheric distillation or two stage atmospheric/vacuum distillation also contains the bulk of the crude components which deposit as carbonaceous or coke-like material on cracking catalysts without substantial conversion. These are frequently referred to as "Conradson Carbon" from the analytical technique of determining their concentration in petroleum fractions.
In the past, and to a certain extent under present operating schemes, high molecular weight stocks containing sulfur, nitrogen and metals have often been processed in a coking unit to effectively remove metals and some of the sulfur (these contaminants remaining in the solid coke). However, there are limits to the amount of metals and sulfur that can be tolerated in the produced coke if it is to be saleable. Hence, considerable effort has been expended for effecting the removal and recovery of metallic and non-metallic contaminants from various fractions of petroleum oils so that conversion of such contaminated charges to more desirable products may be effectively accomplished.
Typically, metals and sulfur removal has been accomplished by the use of hydrotreatment or hydroprocessing. Such hydrotreatment involves the utilization of hydrogen in conjunction with a catalyst comprising a Group VIIIA metal, (or metal oxide or metal sulfide or metal oxysulfide), e.g., Fe, Co, Ni, etc. and a Group VIA metal, (or metal oxide, or metal sulfide, or metal oxysulfide), e.g., Mo, W, etc. deposited on a porous refractory support, e.g., alumina. Among such catalysts, cobalt-molybdenum, or nickel-cobalt-molybdenum supported on alumina are considered to be preferred catalysts because they exhibit good activity for desulfurization, demetalation and for CCR reduction. The pore size distribution of these catalysts was determined to be an important parameter in ascertaining their demetalating and desulfurizing activities. Generally for a given catalyst pore volume, large pore catalysts possess greater demetalating activity than small pore catalysts; small pore catalysts generally posses greater desulfurizing activity than large pore catalyst. There are a great number of patents covering hydrotreating and the following U.S. patents are representative of the art: U.S. Pat. Nos. 3,876,523; 3,931,052; 4,016,067; 4,054,508; and 4,082,695. U.S. Pat. No. 3,684,688 describes a process for increasing normal liquid hydrocarbon yields from coking a hydrocarbon feed.
The prior art has been primarily concerned with hydrotreating coker feed to remove metals and sulfur with less attention afforded to CCR reduction and conserving hydrogen. Thus, the preferred catalysts such as CoMo/Al.sub.2 O.sub.3 and NiMo/Al.sub.2 O.sub.3 are active desulfurizers and as such require a large consumption of hydrogen for desulfurization purposes, thus necessitating additional hydrogen for CCR reduction. In general, non-metallic heteroatom, e.g., sulfur, nitrogen and oxygen, removal accounts for about 5-20% of the hydrogen consumed in residuum hydroprocessing. In stocks such as tar sands bitumen where the heteroatom content is even greater, the hydrogen requirement for heteroatom removal is thus increased.
As previously stated, pore size distribution is an important hydroprocessing catalyst parameter with desulfurization catalysts usually designed to have small pore sizes, e.g., an average pore size of about 100 Angstroms and less. This small pore size permits a high surface area at a given catalyst pore volume. Since the majority of the hydrocarbon molecules which contribute to CCR are large asphaltenic types, conventional small pore desulfurization catalysts would diffusionally restrict the CCR-type materials from being hydrogenated. As a result, the hydrogenation of hydrocarbons is preferentially accomplished on the smaller molecular size portion, and hydrogen consumption is not efficiently utilized for achieving a reduction in CCR.
It is one object of the present invention to provide means to selectively reduce CCR in a residual fraction without needlessly consuming addition hydrogen for other functions, e.g. desulfurization and demetalation. It is another object of this invention to provide means to upgrade residual fractions for use in coker units to reduce coke make.